Generally in video equipment using liquid crystals, for example, liquid crystal televisions or liquid crystal display apparatuses, images are displayed by light emission of backlights composed of cold cathode-ray tubes. Inverters are used so as to generate driving signals for controlling and driving the backlights.
Generally in order to control the luminances of backlights, there are current or voltage dimming systems for changing input DC voltages or input DC currents of inverters to change currents flowing in cold cathode-ray tubes and PWM dimming (burst dimming) systems for controlling light emission and extinction at an oscillation frequency by PWM (Pulse Width Modulation) pulses.
The current or voltage dimming systems have advantages in that noises (audible sounds) generated from the inverters are small and stable but have disadvantages in that the dimming ranges are narrow.
On the other hand, the PWM dimming systems have advantages in that the dimming ranges are wide and are used in the functions of adjusting backlights by users, for example.
Generally, PWM pulse signals for driving inverters are asynchronous with the vertical synchronization frequency of video signals to be inputted. When the PWM pulse signals are asynchronous with the vertical synchronization frequency, however, there occur phenomena in which noises move on screens or phenomena in which interference stripes appear on screens.
Therefore, it has been proposed that the PWM pulse signal is synchronized with the vertical synchronization frequency (see Patent Document 1, for example).
When the frequency of the PWM pulse signal is set to integral multiples of the vertical synchronization frequency, however, a flicker is generated due to the change in time of the light transmittance of a liquid crystal (see Patent Document 2). In order to prevent the generation of the flicker, there is provided a PWM dimming driving circuit that causes a fluorescent tube to flicker m times (m is an integer more than n and other than multiples of n) during a display period for n pictures (n is an integer of not less than 2) in a liquid crystal display apparatus with a backlight control function in the Patent Document 2.
[Patent Document 1] JP 11-126696 A
[Patent Document 2] JP 7-325286 A